HMGB1 and primary progressive multiple sclerosis: When MS patients were stratified according to the different clinical forms, the highest HMGB1 serum levels were observed in patients with RRMS, and differences were statistically significant compared to PPMS patients (P = 5 × 10−5), SPMS patients (P = 0.001), and healthy controls (P = 0.001) (Figure 2B).